#include <string.h>
#include <stdint.h>

static char *twobyte_strstr(const unsigned char *h, const unsigned char *n)
{
    uint16_t nw = n[0]<<8 | n[1], hw = h[0]<<8 | h[1];
    for (h++; *h && hw != nw; hw = hw<<8 | *++h);
    return *h ? (char *)h-1 : 0;
}

static char *threebyte_strstr(const unsigned char *h, const unsigned char *n)
{
    uint32_t nw = (uint32_t)n[0]<<24 | n[1]<<16 | n[2]<<8;
    uint32_t hw = (uint32_t)h[0]<<24 | h[1]<<16 | h[2]<<8;
    for (h+=2; *h && hw != nw; hw = (hw|*++h)<<8);
    return *h ? (char *)h-2 : 0;
}

static char *fourbyte_strstr(const unsigned char *h, const unsigned char *n)
{
    uint32_t nw = (uint32_t)n[0]<<24 | n[1]<<16 | n[2]<<8 | n[3];
    uint32_t hw = (uint32_t)h[0]<<24 | h[1]<<16 | h[2]<<8 | h[3];
    for (h+=3; *h && hw != nw; hw = hw<<8 | *++h);
    return *h ? (char *)h-3 : 0;
}

#define MAX(a,b) ((a)>(b)?(a):(b))
#define MIN(a,b) ((a)<(b)?(a):(b))

#define BITOP(a,b,op) \
 ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))

static char *twoway_strstr(const unsigned char *h, const unsigned char *n)
{
    const unsigned char *z;
    size_t l, ip, jp, k, p, ms, p0, mem, mem0;
    size_t byteset[32 / sizeof(size_t)] = { 0 };
    size_t shift[256];

    /* Computing length of needle and fill shift table */
    for (l=0; n[l] && h[l]; l++)
        BITOP(byteset, n[l], |=), shift[n[l]] = l+1;
    if (n[l]) return 0; /* hit the end of h */

    /* Compute maximal suffix */
    ip = -1; jp = 0; k = p = 1;
    while (jp+k<l) {
        if (n[ip+k] == n[jp+k]) {
            if (k == p) {
                jp += p;
                k = 1;
            } else k++;
        } else if (n[ip+k] > n[jp+k]) {
            jp += k;
            k = 1;
            p = jp - ip;
        } else {
            ip = jp++;
            k = p = 1;
        }
    }
    ms = ip;
    p0 = p;

    /* And with the opposite comparison */
    ip = -1; jp = 0; k = p = 1;
    while (jp+k<l) {
        if (n[ip+k] == n[jp+k]) {
            if (k == p) {
                jp += p;
                k = 1;
            } else k++;
        } else if (n[ip+k] < n[jp+k]) {
            jp += k;
            k = 1;
            p = jp - ip;
        } else {
            ip = jp++;
            k = p = 1;
        }
    }
    if (ip+1 > ms+1) ms = ip;
    else p = p0;

    /* Periodic needle? */
    if (memcmp(n, n+p, ms+1)) {
        mem0 = 0;
        p = MAX(ms, l-ms-1) + 1;
    } else mem0 = l-p;
    mem = 0;

    /* Initialize incremental end-of-haystack pointer */
    z = h;

    /* Search loop */
    for (;;) {
        /* Update incremental end-of-haystack pointer */
        if (z-h < l) {
            /* Fast estimate for MAX(l,63) */
            size_t grow = l | 63;
            const unsigned char *z2 = memchr(z, 0, grow);
            if (z2) {
                z = z2;
                if (z-h < l) return 0;
            } else z += grow;
        }

        /* Check last byte first; advance by shift on mismatch */
        if (BITOP(byteset, h[l-1], &)) {
            k = l-shift[h[l-1]];
            if (k) {
                if (k < mem) k = mem;
                h += k;
                mem = 0;
                continue;
            }
        } else {
            h += l;
            mem = 0;
            continue;
        }

        /* Compare right half */
        for (k=MAX(ms+1,mem); n[k] && n[k] == h[k]; k++);
        if (n[k]) {
            h += k-ms;
            mem = 0;
            continue;
        }
        /* Compare left half */
        for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
        if (k <= mem) return (char *)h;
        h += p;
        mem = mem0;
    }
}

char *strstr(const char *h, const char *n)
{
    /* Return immediately on empty needle */
    if (!n[0]) return (char *)h;

    /* Use faster algorithms for short needles */
    h = strchr(h, *n);
    if (!h || !n[1]) return (char *)h;
    if (!h[1]) return 0;
    if (!n[2]) return twobyte_strstr((void *)h, (void *)n);
    if (!h[2]) return 0;
    if (!n[3]) return threebyte_strstr((void *)h, (void *)n);
    if (!h[3]) return 0;
    if (!n[4]) return fourbyte_strstr((void *)h, (void *)n);

    return twoway_strstr((void *)h, (void *)n);
}
